The rotors for such a machine comprise a female rotor provided with helical lands and intervening grooves having generally concave flanks at least the major portions of which are located inside the pitch circle of the rotor, and a male rotor provided with helical lands and intervening grooves having generally convex flanks at least the major portions of which are located outside the pitch circle of the rotor.
In most cases each land of the female rotors extends a short distance ouside the pitch circle forming an addendum and each groove of the male rotor extends a short distance inside the pitch circle forming a dedendum corresponding to said addendum. The flanks of the addendums and dedendums form minor portions of the complete flanks of the corresponding lands and grooves. Up to now each of said minor flank portions has been shaped so as to follow a circular arc having its centre on the pitch circle. In order to guarantee non-interference between cooperating minor flank portions the radius of the circular arc of the male rotor has been made considerably larger than that of the female rotor, said difference between the radii being of the order of 1 to 1%. Owing to the short extent of said minor flank portions and their location close to the pitch circles where the sliding between the flanks is a minimum, the leakage areas so obtained have been negligible.
Screw compressors having such flank profiles of the addendums have run quite satisfactorily when the rotors have been interconnected by means of synchronizing gears. Also in screw compressors with direct contact between the flanks such addendum and dedendum profiles have worked satisfactorily when the male rotor has been connected with a power source even though the torque transmitting surfaces of the major flank portions adjacent the pitch circles have been of limited size. Those surfaces have normally, in order to obtain flank contact adjacent to the pitch circles only, been provided with a smaller interflank clearance than that of the remaining flank potions in the way well known since the early thirties from the screw rotor pump field. In screw compressors with direct flank contact and the female rotor connected with a power source and using addendum and dedendum profiles of the earlier known type, some problems have arisen partly owing to the larger torque to be transmitted between the rotors and partly owing to the fact that the points of contact of the driving female rotor flank lie inside the pitch circle whereas the points of contact of the driven male rotor flank lie outside the pitch circle, which means that at the points of contact the peripheral speed of the driving flank is lower than that of the driven flank.
Another disadvantage with the described known addendum and dedendum flank profiles is that they are complicated to manufacture and almost impossible to cut properly by a hob milling process.
One object of the present invention is to extend the area of the power transmitting flank surfaces of a pain of intermeshing rotors.
Another object of the invention is to shape the rotors such that at least a portion of the power transmitting surfaces are so located in relation to the respective pitch circles that the driving flank has a higher speed than that of the driven flank.
A further object of the invention is to shape the rotors so that they can be easily cut in a hob milling machine.